1. Field of the Invention
The present invention relates to a control method for establishing a permanent virtual connection and, more particularly, to a control method for establishing a permanent virtual connection in an ATM network.
Standardization of Asynchronous Transfer Mode (ATM) switching has been progressed by organizations such as ITU-T or the ATM forum in order to establish a technology for constructing a Broadband aspects of Integrated Service Digital Network (B-ISDN). It is desired for the B-ISDN to provide more reliable services by maintaining a connection with a particular party and allowing a restart of a communication in a short time when a fault occurs in the network.
2. Description of the Related Art
Conventionally, in an ATM network, a Permanent Virtual Connection is used for a connection between a subscriber and a subscriber exchange and a connection between exchanges in the network. A Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI), which are used or identifying a Virtual Channel and a Virtual Path, are determined when a connection is established between paths, subscribers and exchanges. The VPI and VCI are identifier provided in a header of an ATM cell which has fixed length. The VPI and VCI are used for distinguishing a cell transferred along a connection from a cell transferred along a different connection. Information with respect to paths and the VPI and VCI is managed as a database of a network monitor. Each device located on the paths which constitute a network is provided with data, as a service order, from the network monitor.
FIG. 1 is an illustration of a structure of an ATM network. In FIG. 1, an exchange 10 is connected to each of the exchanges 12 and 13 by a path, and each of the exchanges 12 and 13 is connected to an exchanger 11 by a path. Each of the exchanges 10-13 is connected to a network monitor 14 so that the service order is provided to the exchanges 10-13 and notification is sent from the exchanges 10-13 to the network monitor 14. When a connection is made between subscribers 15 and 16 by the PVC, a route following the exchangers 10, 12 and 11, in that order, is established, for example. In this case, permanent path connection management information shown in FIGS. 2 and 3 is registered in the database of the network monitor 14.
In the conventional Permanent Virtual Connection, the paths and the VPI and VCI are previously determined, and are managed as a database. Thus, when an incommunicable condition occurs in a part of the network and when the connection is reestablished to restart a communication, there are problems described below. It should be noted that the incommunicable condition herein includes a temporary stop of service of a particular connection line or an interface due to a maintenance as well as a fault of a trunk line between exchanges and a stop of service due to system down of an exchange.
(1) In order to determine whether or not a connection is influenced by an incommunicable state in a network, that is, to determine whether or not reestablishment of a connection is required, the network monitor must be provided with trunk line position information of each exchange or discrimination information of each trunk exchange shown in FIG. 2 so as to discriminate each of the permanent virtual connections. This increases an amount of data to be managed for a single connection. In FIG. 2, the location information and the VPI and VCI are parenthesized. The values of the location information and the VPI and VCI correspond to that shown in FIG. 1.
(2) In order to announce to the network monitor the location where the incommunicable condition occurs, all of the devices constituting the network must be provided with the same interface regardless of their scale, or the network monitor must have a function to interpret interface information of each of the devices. In either case, versatile and detailed protocol is needed to put in practice, and a lot of time is required to achieve such a scheme.
Even if the above-mentioned problems are solved and the connection influenced by the incommunicable condition can be discriminated, there are problems as described below when an attempt is made to restart a communication, that is, to reestablish the connection.
(3) In order to reestablish the connection, it is required to select paths to detour the location of the incommunicable condition and the corresponding VPI and VCI. This requires a search for selectable paths for all detours. It is possible that the paths selected as a detour pass a different trunk exchange. Accordingly, the network monitor must have information with respect to the entire structure of the network and usable connection paths which are obtained by combinations as shown in FIG. 3. Thus, a large data base is needed, and a high-speed search logic is needed for minimize a time period of interruption of service.
(4) When a selection of paths is made for reestablishing the connection as mentioned in item (3), it is required for the network monitor to check if bandwidth, which is necessary for the connection on each path constituting the selected path and each device in the network, is assignable. However, there is a case in which the check of the network monitor cannot be performed since a Selection Virtual Connection (SVC) may be established along the paths, devices and trunk lines. Generally, a band assigned to the SVC is not monitored by the network monitor.
(5) In order to eliminate the problem mentioned in item (4), there is a method in which a trunk line or an application for each VPI in the trunk line is separated to classify into one for the PVC and one for the SVC. Even when such a method is used, the network monitor must know a calculation method of allocating a band for each of the devices since the calculation method may differ from device to device. That is, there is a problem in that the network monitor must calculate by an appropriate calculation method for each device which constitutes the selected path which condition is not preferable for a practical operation and operational speed. The calculation method for allocating bandwidth is dependent on the system for transmitting a cell in the device, and it is known that a plurality of systems are present.
As different methods, there are procedures such as an Automatic Protection Switch (APS) and a soft PVC as a means for switching a communication channel at a short time.
(6) The APS requires a complete spare channel. Accordingly, a cost is increased since the spare channel cannot be used in a normal condition. Additionally, since the APS procedure itself is a switching procedure defined on a specific frame format such as SONET or SDH, there is a problem that the APS procedure cannot be used for an interface having a frame format other than the specific frame format.
(7) With regard to the soft PVC, there is a problem in that all of the ATM exchanges constituting the network must be provided with a function to control the soft PVC.